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Monolayer Fibers on Solid Surfaces Made of a Quinone Sulfonate Christian Messerschmidt,† Andrea Schulz,† Jo¨rg Zimmermann,‡ and Ju¨rgen-Hinrich Fuhrhop*,† Freie Universita¨ t Berlin, FB Biologie, Chemie, Pharmazie, Institut fu¨ r Chemie/Organische Chemie, Takustrasse 3, D-14195 Berlin, Germany, and Humboldt-Universita¨ t zu Berlin, Institut fu¨ r Physik, Invalidenstrasse 110, D-10115 Berlin, Germany Received January 11, 2000. In Final Form: April 12, 2000 2-Octadecylquinone-5-sulfonic acid (1) formed orthogonal molecular monolayers on the Langmuir trough, which were reduced to long-lived anions with dithionite added to the subphase. After the smooth films were transferred to different substrates, quantitative rearrangement to long fibers of flat-lying molecules was observed. The fibers formed typical disclinations known from liquid-crystalline compounds. Addition of a β-tetraethyl-β-tetrapyridinyl-porphyrin (2) mixed monolayer, containing flat-lying as well as uprightstanding porphyrin domains, led to selective fluorescence quenching. Only the flat-lying porphyrins (λmax ) 645 nm) were reached by the quinone fibers; the upright-standing porphyrins (λmax ) 660 nm) continued to fluoresce.
Introduction Quinones act as electron acceptors and shuttles in the membrane systems of photosynthesis and respiration. Model systems1 have used the shuttle action of quinones in artificial vesicle membranes.2-5 Asymmetrical vesicle membranes using polar quinones as headgroups6 and surface monolayers on water are also known.6-9 We report here for the first time on the rearrangement of monolayers made of the quinone 1, used in photographic emulsions, upon transfer to solid surfaces. Novel ultrathin fibers are formed which photoreacts regioselectively with attached porphyrin monolayers. Experimental Section 2-Octadecylquinone-5-sulfonic acid (1) was kindly supplied by Agfa-Gevaert (Leverkusen, Germany). 1H NMR spectra and elemental analysis were in agreement with expectations. The synthesis of β-tetraethyl-β-tetrapyridinyl-porphyrin (2) was described earlier.10,11 As the solubility of 1 in chloroform is not very high, the compound was solubilized in a small amount of either DMSO or pyridine. Then the solution was diluted with a 100-fold excess of chloroform. The solutions were spread on the air-water interface on a LAUDA FW2 Langmuir film balance (LaudaKo¨nigshofen, Germany). The subphase water was purified by a * Corresponding author. † Freie Universita ¨ t Berlin. ‡ Humboldt-Universita ¨ t zu Berlin. (1) (a) Gust, D. Top. Curr. Chem. 1991, 159, 103. (b) Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (2) Futami, A.; Hurt, E.; Hanska, G. Biochim. Biophys. Acta 1979, 547, 583. (3) Futami, A.; Hanska, G. Biochim. Biophys. Acta 1979, 547, 597. (4) Leidner, C. R.; Liu, M. D. J. Am. Chem. Soc. 1989, 111, 6859. (5) Siggel, U.; Hungerbu¨hler, H.; Fuhrhop, J.-H. J. Chim. Phys. 1987, 84, 1055. (6) Fuhrhop, J.-H.; Hungerbu¨hler, H.; Siggel, U. Langmuir 1990, 6, 1295 (7) Naito, K.; Miura, A.; Azuma, M. J. Am. Chem. Soc. 1991, 113, 6386. (8) Kruk, J.; Strzalka, K.; Leblanc, R. M. Biochim. Biophys. Acta 1993, 1142, 6. (9) Mindyuk, O. Y.; Heiney, P. A. Adv. Mater. 1999, 11, 341. (10) Endisch, C.; Fuhrhop, J.-H.; Buschmann, J.; Luger, P.; Siggel, U. J. Am. Chem. Soc. 1996, 118, 6671. (11) Donner, D.; Bo¨ttcher, C.; Messerschmidt, C.; Siggel, U.; Fuhrhop, J.-H. Langmuir 1999, 15, 5029.
Millipore system. To reduce the film on the water surface, we used a 10-3 M solution of sodium dithionite (Aldrich; used as received) for the subphase. The LB (Langmuir-Blodgett) films were transferred onto the surfaces at a speed of 1 mm/min. The surfaces used were freshly cleaved mica (Plano, Wetzlar, Germany), silicon wafers (Aurel), and gold surfaces, which were modified according tthe stipulations of Wagner et al.12 Scanning force microscopy (SFM) measurements were performed using a Digital Instruments Nanoscope IIIa (Santa Barbara, CA) in tapping mode. All images were taken under ambient conditions. Height and phase images were recorded simultaneously. The tapping-mode phase offset was always zeroed before scanning. Tapping conditions can be characterized by the rSp value as rSp ) ASp/A0. ASp is the amplitude in contact with the sample, and A0 is the free amplitude. Light tapping corresponds to a rSp value greater than 0.9; hard-tapping conditions have a value smaller than 0.8. Silicon cantilevers (Digital Instruments) with spring constants of 45-60 N/m and resonance frequencies of 270-350 kHz were used. The scanning rate was usually 1.0 Hz, except for the high-resolution image in Figure 5a, which was taken at 12.2 Hz. UV-vis spectroscopy on the Langmuir trough was performed in the transmission mode using a LAMBDA 16 spectrometer (Perkin-Elmer) equipped with a light-conducting device. A silver mirror (Spindler & Hoyer) underneath the water surface reflected the light beam. Steady-state fluorescence of porphyrin monolayers on gold was determined with a cooled CCD-matrix with a mounted spectrometer (Oriel L.O.T. Instaspec IV). The sample was oriented on a five-axis positioning system (Fostec DC 300). Excitation occurred via an Ar+ laser (100 mW, 30-µm-size spot on sample) at 488 nm at an incident angle of 45°. Emitted light was collected perpendicular to the gold electrode surface. For further details, see Korth et al.13 The probes for the fluorescence measurements were prepared in the following way: Three-quarters of a long mica sheet was immersed in the Langmuir trough subphase, acidified by HCl. Porphyrin 2 was dissolved in chloroform and spread onto the Langmuir trough. The mica sheet was withdrawn from the subphase, leaving one-quarter immersed. The porphyrin film was carefully removed and the last quarter of the mica sheet withdrawn from the subphase. Then, after the quinone 1 film was spread onto a new water subphase, half of the mica sheet (12) Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867. (13) Korth, O.; Hanke, T.; I., R.; Ro¨der, B. Exp. Techn. Phys. 1996, 41, 25.
10.1021/la000030f CCC: $19.00 © 2000 American Chemical Society Published on Web 06/03/2000
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Figure 1. (a) Langmuir isotherms of 2-octadecylquinone-5-sulfonic acid 1 (right curve) within 5 min after spreading and (left curve) after 30 min. (b) Uv-vis spectra of the monolayer of quinone 1 on water (lowest trace) before reduction and (upper traces) upon addition of sodium dithionite (10-1 M) to the subphase after 30 s and 10, 20, and 30 min (from the bottom to the top, respectively) in air. was immersed through the film. By this procedure, we obtained a mica sheet with four different coatings: (1) no coating, (2) porphyrin 2 only, (3) porphyrin 2 under quinone 1, and (4) quinone 1 only. It was thus possible to switch between the different quarters during fluorescence measurements using a positioning system. Effects of different thicknesses of the mica sheets were therefore canceled.
Results Quinone sulfonate 1 was spread on water from pyridine/ chloroform solutions, and it produced stable surface monolayers with a final molecular area of 0.17 ( 0.01 nm2. This value was reached within 10 min and did not change within an hour. Freshly spread monolayers produced a value of 0.24 ( 0.01 nm2 per molecule. Both values are in agreement with those of upright-standing molecules, where the quinone moieties finally pack tightly, but a value of 0.17 nm2 is less than one would expect for a sulfonate. The absorption band of the quinone chromophore at 260 nm could not be detected with a fiberoptic spectrometer, but upon addition of dithionite to the subphase, the classical reduction products of quinones became observable. Absorption bands at 400 and 680 nm point to semiquinone radicals, and a 480 nm peak may be attributed to quinhydrone dimers. All absorption bands were broad due to the character of a solid-state radical and were surprisingly long-lived. After 30 min, no apparent decrease of the absorption bands was observed in the presence of air (Figure 1). Transfer of the monolayer from water to mica platelet surfaces allowed the application of scanning force microscopy (SFM). Instead of the expected smooth and rigid monolayer, we observed exclusively 12.0 nm fibers with a length of several micrometers (Figure 2a). The fibers were arranged in a parallel manner, disrupted by singularities known as Schlieren textures or disclinations.14 Panels b and c of Figure 2 show two typical examples with “strengths s” of disclinations of -1/2 and +1 (φo ) π/2). Similar angular distributions are known from liquid crystal structures. The height histogram shows that the fibers have only a height of 0.6 ( 0.1 nm. The uprightstanding, continuous monolayer of 1 on water has been transformed to an assembly of fibers in which the alkyl chains lie flat on the mica surface (see discussion, Figure 8). (14) Demus, D.; Richter, L. Textures of Liquid Crystals; VCH: Weinheim, Germany, 1978.
Transmission electron microscopy (TEM) was then performed on the water surface films. For this purpose, carbon-coated grids were placed on the bottom of a Langmuir trough, and the subphase was removed, thus lowering the surface film to the grid’s surface. Planar monolayers having some crystalline patterns with a spacing of 3.5 nm were observed (Figure 3). No fibers were detectable. We then used silicon and gold in the LB-transfer experiment. Direct SFM height measurements were not successful on these surfaces, but phase imaging clearly differentiated the soft fibers from the hard subphases15 (Figure 4). A closer look on the fibers revealed a fine structure perpendicular to the fiber long axis (Figure 5a,c). The striations became visible at higher magnification and slow scanning (0.5 Hz, Figure 5c) and in the phase image at faster scanning rates (12.2 Hz, Figure 5a). Besides the 0.6 nm high alkyl chains, we also observed structures that were longer by 0.2-0.3 nm and that gave higher phase contrasts. They were alternating with lower parts along the fiber. A profile (Figure 5b) along the line in Figure 5a showed that the distance between two high structures was 7.2 ( 0.2 nm (averaged over five structures). The high structures were assigned to the quinone rings. As pointed out before, the harder parts of the probe (i.e., the quinone rings) gave higher phase contrasts under the light tapping conditions. Figure 5c shows the 12 nm fibers together with the regular striations and many holes, which are less apparent in the overview of Figure 1. We then formed a photoactive, bilayered LB-film system using fibers made of 1 as an electron acceptor and β-tetraethyl-β-tetrapyridinyl porphyrin (2) monolayers as donors.10,11,16,17 At pH ) 2.5, the porphyrin 2 forms LBfilms on mica, containing upright-standing stripes surrounded by flat-lying porphyrin dimers.17 The uprightstanding porphyrins fluoresce at 660 nm, and the flatlying dimers fluoresce at 645 nm. Figure 6a shows the fluorescence spectrum of a film at pH 2.5 compared to that of a film transferred at pH 7. (15) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, 385. (16) Messerschmidt, C.; Schulz, A.; Rabe, J. P.; Simon, A.; Marti, O.; Fuhrhop, J.-H. Langmuir 2000, 16, 1299. (17) Endisch, C.; Bo¨ttcher, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1995, 117, 8273.
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Figure 2. Scanning force micrographs on mica. (a) 5 µm image in height mode. (b,c) Particular disclinations in the transferred film compared with structures from the literature.14 (d) Height histogram of the boxed area in (b): the lower peak corresponds to the mica surface and the higher peak to the fibers; the height difference between the two peaks is 0.6 nm.
Adsorbing a quinone 1 LB-film on top of a porphyrin 2 film proved to be difficult. All attempts failed involving
a porphyrin 2 film transferred at pH 7. The porphyrin film pealed off during the transfer of the quinone film.
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force microscopy (AFM) again. Figure 7b shows the characteristic stripes of the underlying porphyrin layer (pH 2.5), which have been characterized before.16 Without quinone, the upright-standing porphyrin stripes have a height of 1.7 ( 0.1 nm, and the surrounding porphyrin dimers are 0.7 ( 0.1 nm high. Measuring height histograms (Figure 7c) and profiles (Figure 7d) at a defect in the porphyrin film, we found a prominent height at 1.3 ( 0.2 nm in the histogram, which can be assigned to the flat-lying porphyrin dimers (0.7 nm) plus the height of the flat-lying quinones 1 (0.6 nm). The difference in the height also implies that no film of porphyrin 2 was adsorbed by the mica surface. The reversed direction of film transfer touching first the hydrophobic alkyl chains does not work. The upright-standing porphyrins make up only a small fraction. They were not detected in the height histograms. The height profile in Figure 7d, however, shows a height of 2.3 nm for them, which is fairly close to the expected values of 1.7 nm for upright-standing porphyrins and 0.6 nm for flat-lying quinones. The fluorescence spectra of the mixed film only showed a band at 660 nm. The 645 nm band of the flat-lying porphyrins was totally quenched (Figure 7f). Discussion
Figure 3. TEM image of a Langmuir-Blodgett film of 1 transferred to a grid lying at the bottom of the Langmuir trough by lowering the subphase. No characteristic structures can be found, except for a few crystals of 1. The spacing in these crystalline spots is 3.5 ( 0.1 nm.
The quinone film on water at pH 2.5 also did not settle on the porphyrin film on mica when the latter was pulled upward through the quinone layer. The porphyrin dissolved instead in the subphase. The only possible way to produce the desired sample was to lower a mica platelet with the porphyrin (pH 2.5) film attached through the quinone layer into the subphase and then to carefully remove the remaining quinone film (Figure 7a). To check whether the quinone 1 film adhered to the porphyrin 2 film without removing it and to investigate how the quinone film rearranged upon transfer, we applied atomic
The first unusual result with the monolayers of quinone sulfonate 1 is the small molecular area of only 1.7 nm2. On water, the corresponding condensed phase is only reached after a long-lasting ripening process, and TEM showed a homogeneous monolayer. We therefore assume a well-ordered arrangement of tightly packed quinones, the presence of hydrogen bridges between water and the highly electronegative quinone oxygen atoms, and close interactions between parallel adjusted quinone moieties. The stability of the quinone radical spectrum in the presence of air indicates that the whole quinone moiety may be covered by the dithionite solution. This detail is also depicted in Figure 8a. A surprising rearrangement then occurred upon transfer to solid surfaces: 12-nm-wide fibers with 7.2 nm striations (Figure 5c) and heights of 0.6 and 0.9 nm were found exclusively (Figure 8a). The 0.6 nm height clearly points to flat-lying monomers. The molecules fell flat on mica as well as on gold and silicon. If one assumes 0.6 nm
Figure 4. AFM phase images of a Langmuir-Blodgett film of quinone 1 transferred onto (a) gold and (b) silicon. The left image has been taken under hard tapping conditions, so the contrast is reversed.
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Figure 5. High-resolution images of a Langmuir-Blodgett film of quinone 1 transferred on mica revealing a fine structure perpendicular to the fiber long axis. (a) Phase image of a 140 nm scan at 12.2 Hz. (b) Profile along the line drawn in (a): the arrows are set on structures with a high phase shift 5 repeating units apart; dividing the resulting horizontal distance of 35.9 nm by 5 gives a length of 7.2 ( 0.2 nm for one repeating unit. (c) Height image at 0.5 Hz.
for the width of an alkyl chain, then about 20 molecules lying side-by-side would be needed to yield a 12-nm-wide fiber. The molecular area of flat-lying amphiphiles is about 1.0 nm2, as calculated from a molecular model. The cross sectional area of molecules in an upright arrangement is only 0.2 nm2. The calculated transfer ratio should thus be
in the order of 0.2 nm. This number, however, could not be determined experimentally. The surface monolayer on water was not stable enough. The 7.2 nm striations would point to a molecular bilayer (=6.0 nm) plus an ion-filled water layer (=1.2 nm). The overall structure would be similar to that of an extended bimolecular ribbon taken
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Figure 6. Comparison of the fluorescence spectra of Langmuir-Blodgett films of Porphyrin 2 at (a) pH 2.5 and (b) pH 7. Insets: AFM images of the corresponding films. The characteristic stripes in (a) have been characterized in an earlier publication.16
Figure 7. Bilayer system of Langmuir-Blodgett films consisting of a layer of quinone 1 on top of a layer of porphyrin 2. (a) Applied LB-technique: a mixed monolayer of upright-standing and flat-lying porphyrin 2 molecules on mica is pushed through the quinone 1 monolayer. (b) AFM image of the porphyrin 2-quinone 1 bilayer film. (c) Height histogram of the inset: The left peak corresponds to the mica surface and the right peak to the flat lying porphyrin (0.7 nm) plus the quinone (0.6 nm). (d) Section analysis along the line drawn in the inset of (c), the 1.4 and 1.5 nm peaks correspond to the flat-lying porphyrin dimers plus quinone; the 2.3 nm peak corresponds to the upright-standing porphyrins plus quinone. (e) Model of the bilayer domains. (f) Fluorescence spectrum of porphyrin 2 monolayers after addition of quinone 1 fibers (compare with Figure 6).
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Figure 8. Model for the arrangement of the molecules. (a) On the Langmuir trough: the molecules take an upright position with the sulfonic acid group immersed in the subphase. (b) In the fibers: double layers of molecule 1 are separated by a zone containing counterions and water from the subphase.
out of a soap bubble. In the surface monolayers on water, the sulfonate group draws the quinone moieties into the water subphase. The keto-oxygens then also become hydrated, and hydrogen bonds enforce tight packing. Upon transfer to solid surfaces (mica, gold, or silicon), the hydrate layer containing the sulfonate and its sodium counterion is retained, and the alkyl chains rotate toward the surface. Flat-lying fibers are thus formed. Their width is limited to 12.0 nm, corresponding to about 20 molecules. The reasons for fiber formation are presumably (i) strong linear binding forces between the quinone moieties, (ii) limitation of fiber width by sulfonate repulsion and hydration forces,18,19 and (iii) the larger width of alkyl chains compared to quinone rings (Figure 8b). Fluorescence quenching with the cationic porphyrin layer only occurred if the anionic quinone fiber came into (18) Mo¨hwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441 (19) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171
direct contact with the porphyrin chromophore, not when it attached to the peripheral pyridinium units (see Figure 6). This observation is in accordance with the earlier results of fluorescence quenching in porphyrin fibers and monolayer platelets. If the distance between quencher and porphyrin is greater than a critical distance of about 0.7-0.8 nm, quenching is inefficient or absent; only at distances smaller than 0.6 nm may quenching be expected.20 Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 448 “Mesoscopically Structured Systems” and SFB 312 “Vectorial Membrane Processes”) and the FNK of the Free University. LA000030F (20) Fuhrhop, J.-H.; Ko¨ning, J. Membranes and Molecular Assemblies: The Synkinetic Approach; Royal Society of Chemistry: Cambridge, 1994.
